1. Field of the Invention
The present invention relates to an outboard motor which can emit an exhaust gas from an engine into water.
2. Description of the Related Art
Outboard motors are provided with an exhaust passage through which an exhaust gas from an engine can be emitted into water. Typically, the exhaust passage is formed through a boss of a propeller. An exhaust gas can pass through the exhaust passage in the propeller boss and then be emitted from the rear end of the propeller. See, for example, Japanese Patent No. 2747725 and JP-A-Hei 7-144695.
Meanwhile, the exhaust passage needs to have a size corresponding to the output of the outboard motor. For example, the outboard motor having high output emits a larger amount of exhaust gas from the engine. In this case, an exhaust passage having a larger sectional area is required. Therefore, for example, one outboard motor has a propeller disposed in front of the lower case (Japanese Patent No. 2717975), and another outboard motor is designed to emit an exhaust gas from the upper case rearward of the propeller (JP-B-Hei 7-74033).
In the conventional structure of emitting an exhaust disclosed in Japanese Patent No. 2747725 and JP-A-Hei 7-144695, the exhaust passage is formed in the boss of the propeller. In such structures, however, only a limited passage area is obtained in the propeller boss. Besides, exhaust pressure will necessary increase due to the sectional area of the passage.
In Japanese Patent No. 2717975, since the propeller is arranged in front of the lower case, the propeller may hit against an obstacle in the sea.
In JP-B-Hei 7-74033, since an exhaust outlet is formed above the propeller, the exhaust outlet may be exposed above the water surface, and thus exhaust noise is more likely to escape into the ambient air.